1. Technical Field
Apparatuses consistent with the present invention relate to mercury-free arc tubes for use in discharge lamp units, and more particularly, to mercury-free arc tubes having an increased luminous intensity rise.
2. Description of Related Art
In a related-art discharge lamp unit used as a light source of a vehicle lamp, a discharge bulb has a structure in which an arc tube having a sealed glass bulb forming a sealed chamber as a light emitting portion is integrally formed with an electrically insulating plug body made of a synthetic resin. For example, a rear end portion of the arc tube is supported by a metal support member fixed to the electrically insulating plug body. A front end portion of the arc tube is attached to a metal lead support serving as a current conduction path extending from the electrically insulating plug body.
The related-art arc tube has a structure in which a main light emitting metal halide (e.g., Na, Sc, or the like), mercury, and a starting rare gas (e.g., Xe gas or the like) are enclosed in the sealed glass bulb provided with a pair of electrodes. Light is emitted by an arc generated by an electric discharge between the electrodes.
The mercury in the sealed glass bulb acts as a buffer substance. The mercury keeps the tube voltage constant in order to reduce the amount of electrons colliding with the electrodes to thereby reduce damage caused by the electrodes. Also, the mercury acts as a light emitting substance for emitting white light. However, the related-art discharge lamp unit has a disadvantage in that mercury is a substance which is highly toxic to the environment. In response to the social needs of reducing the cause of global environmental pollution, it is advantageous to develop a mercury-free arc tube.
JP-A-2003-168391 describes a mercury-free arc tube which is able to obtain a characteristic similar to that of a mercury containing arc tube. The related art mercury-free arc tube adopts a configuration in which a main light emitting metal halide (e.g., Na or Sc) and a buffer metal halide e.g., Zn) are enclosed in a sealed glass bulb. The buffer metal halide is selected as a substitute for mercury to serve as a buffer substance. A pressure of an enclosed starting rare gas (Xe gas) is adjusted to be high.
However, the structure described in JP-A-2003-168391 also has some disadvantages. For example, although a luminous flux rise is improved to some extent, the luminous flux rise is slower than that of the mercury containing arc tube. In the mercury containing arc tube, an output of 80% is obtained after four seconds from a time when light is emitted by Hg (i.e., the luminous flux rise is fast), but in the mercury-free arc tube, an output of 25% is obtained after four seconds in the case of using Na or Sc (i.e., the luminous flux rise is slow). In a vehicle head lamp, a luminous intensity rise standard (e.g., 6520 cd or more after four seconds from a lamp-on timing) is set at a certain light distribution point. However, in the related art mercury-free arc tube described in JP-A-2003-168391, the luminous intensity rise of the head lamp using the related art mercury-free arc tube as the light source is slow since the luminous flux rise is slow.